docs: def validate_types(self, types=None):
          """Return a valid list of types.
  
          `types` may be a string or a list of strings.
          If `types` is not supplied, returns the default_types.
  
          """
          types = types or self.default_types
          if types is None:
              types = []
          if isinstance(types, six.string_types):
              types = [types]
          return types